le lait

Aina, all French nouns (proper names/nouns excluded) can be preceded by a definite article - it is an inherent feature of French grammar. 

‘Le lait’ is used to refer either to milk in general -“ le lait est bon pour la santé “,  or some specific milk - “ le lait dans le frigo est frais “. 

I think you may be thinking of the distinction between indefinite and partitive articles ?

https://progress.lawlessfrench.com/revision/glossary/article-type/indefinite-article 

https://progress.lawlessfrench.com/revision/glossary/article-type/indefinite-article